In accordance with Winchester fixed disk drive technology, data transducer head sliders "fly" upon an air bearing effect in very close proximity, e.g. 7 micro inches or less, to a disk data storage surface. The air bearing exists only when the storage disk is rotating. When the disk stops rotating, in a "contact-start-stop" disk drive, the head sliders "land" on the disk surface. Storage media is frequently provided with an overcoat or a lubricating coat in order to withstand direct contact between the head slider and the storage surface. Consequently, some disk drive manufacturers permit the slider to land at any location of the data storage disk.
Direct contact between the disk and the heads may abrade or interfere with the storage media. Data recorded at the location of direct contact may be changed, or a permanent defect known as a "hard error" may develop. Accordingly, many disk drive manufacturers provide a dedicated landing zone for the head sliders. This landing zone is usually selected to be the innermost usable radius of the data storage disk, as magnetic storage cells or domains are fewest at the radially inwardmost area of the disk.
Head sliders are typically formed of highly lapped ferroceramic material. The edges of the rails are very sharp. Radial displacement of the head sliders while in contact with the data storage surface has proven very detrimental to the integrity of the storage surface. Such movements may gouge, scratch or scrape away the magnetic data storage coating. Thus, a latch is frequently provided to lock the head positioner assembly (herein "actuator") at the landing zone when power is removed from the disk drive and/or the spindle motor is not spinning (as may occur during a reduced power standby state).
Actuator latches have taken many forms. One approach pioneered by the assignee of the present invention has been to provide an aerodynamically released actuator latch which releases the actuator in response to airflow generated by disk rotation by overcoming a reverse bias force; see, e.g. commonly assigned U.S. Pat. Nos. 4,538,193; 4,692,829 and 4,647,997. One drawback of the approaches described in these patents is that with small disk diameters, such as 3.5" and below, airflow from a single disk may be insufficient to enable the actuator latch to operate reliably within a manufacturable design for mass production of disk drives.
Bistable electromagnetic latches have been proposed in the prior art. Pertinent examples include U.S. Pat. No. 4,881,139 to Hazebrouck; U.S. Pat. No. 4,654,735 to Izraelev et al.; U.S. Pat. No. 4,965,684 to Stefansky; and U.S. Pat. No. 4,903,157 to Malek. Other patents considered in preparation of the application leading to this patent include U.S. Pat. Nos. 4,890,176 and 4,947,274 to Casey et al.; U.S. Pat. No. 4,868,695 to Quatro et al.; U.S. Pat. No. 4,851,943 to Perry; U.S. Pat. No. 4,764,831 to Patel; U.S. Pat. No. 4,751,595 to Kishi et al.; U.S. Pat. No. 4,706,142 to Hattori et al.; U.S. Pat. No. 4,686,595 to Bryer; U.S. Pat. No. 4,660,120 to Manzke et al.; U.S. Pat. No. 4,139,874 to Shiraishi; U.S. Pat. No. 4,594,627 to Viskochil et al.; and U.S. Pat. No. 4,716,480 to Wiens et al.
Commonly assigned, copending U.S. patent application Ser. No. 07/696,629 filed on May 7, 1991 and entitled, "Bistable Magnetic/Electromagnetic Latch for Disk File Actuator," now U.S. Pat. No. 5,208,713, describes an improved bistable latch employing magnetic and electromagnetic elements for latching a disk file actuator so that data transducer heads are parked and maintained at a predetermined landing zone. Although this device works effectively for its intended purpose, it requires a number of components and is relatively expensive to make. In addition, this prior art latch operated satisfactorily only within a narrow range of power supplied from an external power supply.
Another approach which is pertinent to the present invention is described in U.S. Pat. No. 5,025,355 to Stefansky. This patent describes an actuator latch that employs a fixed capture magnetic assembly for capturing a radially extended, magnetically permeable capture body of the rotary voice coil actuator as the landing zone is approached.
One drawback of the fixed capture magnets is their tendency to exert a bias force upon the actuator as it moves into the vicinity of the parking zone, thereby interfering with servo loop control of the head position at radially innermost tracks. In addition, as drive diameters become smaller, to 2.5" or 1.8" one serious drawback of known magnetic capture latches is that the capture magnet unduly influences the actuator during its range of movement over the storage tracks, leading to head positioning instabilities.
Hence, despite the numerous and varied approaches exemplified by the above patents, hitherto unsolved need has remained to continue to develop effective, low cost mechanisms for latching a disk drive actuator to maintain the heads in the landing zone when the disk is not spinning.